A rash drug rash is the most common type of skin rash in adverse drug reactions. The primary pathogenesis may be immunologic and is often thought to be a cell-mediated hypersensitivity reaction. The overall pathophysiologic mechanism is unclear and may be more complex. The diagnosis of rash drug rash also means the differentiation from other diseases, especially from infectious diseases (mainly viral rash) is particularly important. The following are some of the aspects: a. Incubation period: the incubation period of the drug rash in those who have not been sensitized by the drug is as short as 6 to 7 days and as long as several months, with an average of 7 to 14 days. The rash of those who have been sensitized by the drug can take 6 to 48 hours, mostly 12 to 24 hours. The rash is similar to measles, scarlet fever and other viral or bacterial infections, but without the other symptoms of measles and scarlet fever. The rash has the following characteristics: 1. The rash is bright and shiny: it is red and the rash is shiny. Its sensitivity is 85% and specificity is 82.36%. 2. The distribution of the rash is relatively symmetrical on both sides: the distribution of the rash on both sides of the body is usually disproportionate, with one side being heavier and the rash denser and the other side being lighter and the rash thinner. Its sensitivity is 87.5% and specificity is 85.36%. Infectious disease rashes are usually absolutely symmetrical. However, the rash of scarlet fever can also be relatively symmetrical and can be considered an exception. 3. The rash is sparse and dense: i.e., a localized sparse rash and a dense rash exist together. The sensitivity is 100% and the specificity is 90.9%. The distribution of infectious disease rash on the same site is relatively uniform, even if the uneven distribution can be seen, there is a certain pattern, such as dense proximal end and sparse distal end or the opposite, but there is no sudden change between sparse and dense, and the location is fixed, unlike drug rashes that can be seen everywhere. 4, the rash is scattered and fusion interlaced: drug rash can be seen scattered drug rash and fusion drug rash interlaced on the same site. The sensitivity is 95% and the specificity is 82.6%. Infectious rash diseases can also be seen, but the location is relatively fixed, such as rubella fusion rash is seen in the cheeks and symmetrical, early childhood emergency rash fusion rash is seen in the lumbosacral, only streptococcal A pharyngitis fusion rash without a fixed location, but can see clusters of papules mixed in between, can be distinguished. 5, between the rash puffy phenomenon: drug rash between the skin despite the normal skin color, but there are varying degrees of puffiness, its sensitivity is 85%, the specificity is 82.92%. The vast majority of infectious rash diseases have normal inter-rash skin. 6, the coexistence of multiple forms of rash: the main manifestations (1) scattered and fused erythematous plaques interspersed with windbag-like damage; (2) measles-like maculopapular rash, maculopapular rash with puffiness, small papules, small blistering eczema-like lesions; (3) measles-like rash with purpura-like rash of the lower extremities, but streptococcus A-induced pharyngitis may also see such damage. (4) A mixture of measles-like, eczema-like, and purpura-like lesions, i.e., herpes or purpura. Mostly seen in drug fever, the rash is synchronized with fever. The sensitivity is 27.5% and the specificity is 68.75%. 7, no localized rash migration phenomenon: this refers to the pruritic scattered rash and fusion rash in one place after the receding, and then in another place to reappear, that is, the so-called rash is no localized migration phenomenon. The sensitivity is only 12.5%, but the specificity is 100%, mainly in drug fever, where the rash is synchronized with fever. This phenomenon is different from the rash sequence inherent to infectious diseases. 8, rash extension focused on flexion: usually drug rash rash in the back than the chest dense, waist hip than the abdomen, femoral dense, extensor rash than flexion of the extremities dense, often fused with each other. Its sensitivity is 85% and specificity is 70.02%. The rash is also more dense in areas of pressure and friction. If the rash occurs in the axilla, elbow fossa, groin and N fossa, it can show inter-rash-like with maceration, with a sensitivity of 25% and specificity of 80.02%. 9, paracentral or centripetal rash: paracentral rash is a rash that starts on the head and face and then spreads rapidly to the trunk or extremities, with a sensitivity of 35% and specificity of 62.92%. The centripetal rash is a rash that starts at the wrist or ankle and then develops all over the body toward the trunk. This rash is mostly seen in the rash type drug rash caused by ampicillin, with a sensitivity of 60% and specificity of 92.92%. 10, pruritus: the rash is often accompanied by varying degrees of pruritus. Its sensitivity is 85% and specificity is 82.92. 11, fever: mostly occurs in patients who apply antipyretic and analgesic drugs, often due to errors in clinical judgment, mostly diagnosed as infection fever, body temperature decreases about 0.5 to 1 hour after drug administration, after 2 hours, the temperature rises, with the increase in the number of drugs, the temperature rises to 40 degrees, and the rash appears simultaneously with fever, without headache, nausea, vomiting, and There is no headache, nausea, vomiting, whole-body muscle and joint pain of infection poisoning symptoms, and the body temperature starts to drop after 24-48 hours of stopping the drug. 12, self-limiting course: rash type drug rash develops rapidly, often within 1 to 3 days all over the body, most patients in a few days after discontinuation of the rash began to fade, 1 to 2 weeks can return to normal, and in severe cases can last 4 weeks. The rash may be accompanied by a little flaking when it subsides. However, some patients may develop exfoliative dermatitis and toxic epidermolysis bullosa. If four of the above items are present, and the time from drug use to rash is consistent with the incubation period of drug rash, a clinical diagnosis can be made. Differential diagnosis steps 1. Judging from the rash morphology: First, observe the distribution of the rash over the body – whether it is relatively or absolutely symmetrical. Second, observe the distribution pattern of the two rashes to see if there is sparseness, dispersion fusion interlacing, inter-rash puffiness, polymorphic rash and rash displacement. 2, inferred from the theory: detailed questioning and access to the patient’s medication, such as the name of the drug, dose, method, start and end time and any abnormal sensations and past drug allergy history during the use of the drug. This should generally be traced to 28 days prior to the onset of the rash, or at least to 14 days. The time connection between the drug and the rash should be studied to determine whether the allergic period of drug rash is met. V. Diseases that need to be identified 1. measles and atypical measles 2. scarlet fever 3. rubella 4. early childhood rash 5. enterovirus infection 6. adenovirus infection 7. infectious mononucleosis 8. other rash diseases typhoid fever, typhus, endemic typhus, respiratory fusion cell virus infection, poliomyelitis, mumps, influenza and parainfluenza, and certain chlamydial and mycoplasmal diseases The rash can occur and should be differentiated from these diseases.